WEBVTT

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University of Lille 1 - Faculty of biology

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Animal Biology

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Sea star dissection

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Morphology

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The Asteria rubens sea star

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is an Echinoderm

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belonging to the class of Asteroidea.

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It lives on costs

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we may find it in the intertidal zone

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at low tide.

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It feeds chiefly on mussels

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fixed on rocks.

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The sea star is part of the foreshore food web

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and can fall prey to other animals.

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The body of the sea star is flattened,

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in a shape of a star with 5 arms.

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The arms merge at their base

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and are attached to the flat disk.

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The arms are called radius

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The zones between the arms are called interradius.

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This organisation is representative of the pentaradial symmetry,

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characteristic of the adult echinoderms.

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The sea star has two faces:

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the upper face,

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or aboral surface,

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is convex and pigmented.

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The bottom side

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or oral surface,

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is flattened, and clearer,

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in contact with the substrate.

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This surface includes the mouth,

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located in the middle of the flat disk.

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On the aboral surface,

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at the interradius level,

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we can see a whitish limestone plate,

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the madreporic plate.

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There are bony spines convering the body,

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organized in rows.

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On the aboral surface,

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there is a row in the middle of each arm

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and two or three lateral rows.

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By turning the sea star over,

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we can see a supramarginal row

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and an inframarginal row

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Two rows of prikly adambulacra

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delimit a central furrow,

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the ambulacral groove,

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Every ambulacral groove has 4 rows of fleshy and movable tubes

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tube feet or podia,

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ended by a sucker.

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At tip of each arms we can see eyespots,

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sensory organs red in coulour

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sensitives to the light.

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Integument

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In the sea star,

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the body and the spines are covered

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by an ciliated epidermis

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The dermis,

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situated beneath the epidermis,

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has calcareous plates,

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or ossicles

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in which the spines are articulated.

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The locations of ossicles

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are visibles on the surface of the animal.

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The integument has specialized structures,

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pedicellariae,

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put in the shape of a crown at the base of the spines.

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In Asteroidae,

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they have a form of a pincer with two jaws.

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Pedicellariae are used

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to clean the integument

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and the defense of the animal

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Organs used for the breathing,

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gills or papules,

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are associated with the integument.

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they are between the ossicles of the skeleton.

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The presence of an epithelium

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ciliated on the inner side of the integument

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enables the circulation of the liquid

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inside the body cavity,

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the coelom.

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Echinoderms have a calcareous internal skeleton,

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formed by ossicles.

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the cross section of the arm enables us

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to observe the organisation of the dermal plates.

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We can see:

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a dorsal ossicle

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two supramarginal plates

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and two inframarginal plates each having spines,

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two adambulacra plates

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and two ambulacral plates.

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Between the ambulacral plates,

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there are pores that allows the passage of the tube feet.

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General anatomy

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To observe the internal organisation of the sea star,

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we have to cut the sea star with scissors.

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Incise the integument

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following the outline

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to separate the upper half

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from the lower half of the animal.

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Identify the madreporic plate

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and avoid it when you incise.

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Lift the integument of the aboral surface

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starting from the end of the arms

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and cut carefully the underlying organs

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ending by the flat disk.

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The rest of the dissection will be done in the water.

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In the coelom we can see gonads

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and a part of the digestive system.

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The digestive system

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The digestive system includes the mouth,

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the stomach divided in two superimposed chambers,

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the pyloric ceca and the rectal ceca.

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It ends by a microscopic anus,

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located in the aboral surface.

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The observation of the digestve system

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will be done using the aboral portion.

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In the central part of the disk,

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we notice two rectal ceca.

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Below we notice the upper chamber of the stomach,

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the pyloric portion with a pentagonal shape.

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5 canals start from this chamber

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that extend themselves in the arms of the star.

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Each pyloric canal is connected to two pyloric ceca

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that take up a large part of the arms.

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The pyloric ceca,

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composed by dozens of diverticula on dead end

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play a digestive key role

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and are used to the storage of nutrients.

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To observe the rest of the digestive system

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we have to remove the rectal ceca,

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pyloric ceca

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and the pyloric stomach.

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The ventral chamber or cardiac stomach,

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has a shape of five lobes rose.

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For the ingestion of preys,

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this part of the stomach is everted.

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After the external digestion,

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the stomach gets back to its position

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thanks to the contraction of five pairs of retractor muscles

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put in the ambulacral plates.

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To see the mouth,

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we have to detach the retractor muscles

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and remove the portion of cardiac stomach.

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The mouth,

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located in the middle of the flat disk,

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surrounded by a membrane,

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the peristomium.

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Reproductive system

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In Asteroidea

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the sexes are separated

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but there is not a sexual dimorphism.

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Gonads are located in the coelom.

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To observe the gonads

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we have to remove the pyloric ceca,

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and the upper chamber of the stomach.

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Every gonad,

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with a granular apsect,

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is divided in deux lobes

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that all extend in two adjacent arms.

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The size of gonads is variable,

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it depends on the state of sexual maturation of the animal.

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Females and males produce gametes

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through genital pores located in the interradius

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at the junction with the flat disk.

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The fecondation is extern.

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Aquifer system

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One of the characteristic of Echinoderms

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is the existence of an aquifer system.

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It is involve in the locomotion and the food intake.

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This system have a network of conduits

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filled with a liquid

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whose composition is close to the sea water

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The aquifer system have a madreporic plate,

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the stone canal,

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the ambulacral ring,

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five radial canals,

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and many tube feet or podia.

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To observe the different parts of the aquifer system

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we have to open the sea star

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and remove the digestive

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and reproductive system.

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The madreporic plate, or madreporite,

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is visible on the aboral surface of the animal.

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This calcareous plate is riddle with holes

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"aquifer pores"

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that permit the sea water to enter in the system.

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The madreporite is in connection with the madreporic canal

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or stone canal.

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This canal

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sostained by superimposed calcareous rings,

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go down toward the oral surface of the animal.

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The stone canal

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rejoin the ambulacral ring surrounding the mouth.

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This ring is hidden

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with calcareous plates of the skeleton.

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On the circumference of the ambulacral ring of the star

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in the interradial position,

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we notice 9 vesicles with an immune function:

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Tiedemann bodies.

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From the aquifer ring goes in each arm

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an ambulacral canal or radial canal,

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located between the epidermis and the ambulacral plate.

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From the radial canal goes many lateral canals.

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Those are connected to the tube feet or podia.

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The injection of a dye into ambulacral canal of an arm

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enables us to hightlight

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a large part of the aquifer system.

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Podia have three parts:

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ampulla or bulbous ampulla

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a muscular and elongated foot

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and a sucker at the end.

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The podia go through the skeleton at pores level

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located between the two adjacent ambucral plates.

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To observe those pores

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we have to scratch the polian vesicules

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on a part of the arm.

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The contraction of the polian vesicles

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allows the turgescence of the tube feet

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and the the monitoring of the sucker 
for the locomotion of the animal

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The podia within the ambulacral groove

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are protected by rows of movable ambulacral spines

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and by many pedicellariae.

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Radial canals

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end with sensory tentacles

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close to the eyespot.

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Pedagogical design: Pierre-Eric Sautière Jacopo Viziolo

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Technical production: Bernard Mikolajczyk

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Designs: Jacopo Vizioli

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Computer graphics: 3D MEDICUS

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Voice over: Alain Nempont

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Music; Ambient slamming bass jam, Jay Berlinsky

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Gratitude: Sébastien Lefebvre, Michel Priem

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Gratitude: Patrick Flammang, Michel Jangoux

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